Garter Stitch
Reference for the below knit kits:
  


Garter Stitch is created by knitting every row if you are knitting a flat item (scarf or baby blanket kits) or alternating knitting 1 row, then purling 1 row when knitting in the round (cowl kit).

Garter Stitch creates rows of alternating purl bumps that resemble a frown and a smile.  The frowns are slightly above the smiles.

Below are two different methods for weaving in Garter Stitch.  Both methods weave through the smiles and frowns.  Method 1 is easier and recommended for true beginners.  Method 2 is more complex, but hides the ends a bit better.

If you are weaving in ends where you added a ball of yarn, you will have two ends to weave.  Untie the knot you made and criss cross the ends before you start weaving.

If you added your ball in the middle of the row, weave the end that is now on the left, going left and the end that is now on the right going right.  Below is an image showing the ends with the knot undone and then criss-crossing the ends.
   
If you added your ball at the beginning of the row, weave the end on top through the nearest row and weave the end below through the row below.  Below is an image showing the ends with the knot undone and then criss-crossing the ends.
   

Method 1: Weaving in on the Horizontal
Begin by inserting your needle up into the nearest frown and bringing your end through.  I am using a contrast yarn so that it is easier to follow along.

Then, insert your needle down into the adjacent smile and bring your end through.

Repeat this sequence 2-3 more times.  Thread your needle up and through the adjacent frown, then down and through the adjacent smile.
   
Once you have woven your end through a few more frowns and smiles, trim your end leaving about 1/4 of an inch.

Once you stretch your fabric a bit, your 1/4 inch end should disappear into the fabric.


Method 2: Duplicate Stitch Method

With Duplicate Stitch Method, we are weaving along the path of the actual stitches in a row.  Begin, by inserting your needle up and through the nearest smile.

Then, insert the needle down and through through the nearest adjacent smile.

Underneath the smile you just came through, is a flat upside down "V".  Bring your needle parallel and underneath the upside down "V'.  Thread the end through.

Thread the needle up and through the above smile, which you recently came down through.

Repeat this sequence a few more times.  Bring your needle down and through the adjacent smile.

Then, underneath the upside down "V" below

Then, back up the smile above that you recently came down through.

After completing the sequence a few more times, trim the end leaving about 1/4 of an inch.  If you stretch your fabric a bit, the 1/4 inch end should disappear into the fabric.